Near-bit stabilizers are well known in downhole drilling applications and are commonly utilized in conjunction with rotary steerable systems in directional drilling applications. Commonly utilized near-bit stabilizers are typically slightly under gauge and not automatically adjustable. As a result, the stabilizer is offset from the center of the borehole by half the difference between the borehole diameter and the outside diameter of the stabilizer. The direction of offset is variable and generally cannot be predicted. Moreover, the stabilizer is typically free to move and vibrate in the borehole since it is under gauge. These difficulties become more significant in oversize boreholes (e.g., due to washout of a soft formation) and upon stabilizer wear (which decreases the effective diameter of the stabilizer). Large radial vibrations and shock loads are known to occur within this freedom movement. Steering difficulties are also encountered as the near-bit stabilizer no longer pivots predictably. It will therefore be understood that there is a need an adjustable near-bit stabilizer.
Adjustable stabilizer mechanisms are well known and commonly used in downhole tools. Adjustable stabilizers can commonly be classified in one of two groups: (i) those that do not rotate with the drill string and (ii) those that are rotationally fixed to the drill string and therefore rotate in the borehole. Commercially available rotary steerable tools, such as the Pathfinder Energy Services Pathmaker® tool, are examples of stabilizers that may be classified in the first group. The Pathmaker® tool automatically and uniquely adjusts the extension of each of three extendable and retractable blades to maintain the tool at a predetermined offset and direction relative to the center of the borehole. As is well known to those of ordinary skill in the art, rotary steerable tools require an electronic (smart) control system (e.g., including one or more microprocessors as well as numerous electronic sensors) to continually adjust the extension of the blades and to maintain the predetermined offset and direction. While rotary steerable tools are known to provide excellent steerability in certain drilling conditions and automatically adjustable stabilization, they are not suitable for many downhole applications (owing in part to the high cost of such tool deployments).
Adjustable stabilizers that are rotationally fixed to the drill string are also known. For example, U.S. Pat. No. 6,732,817 discloses an expandable underreamer and/or stabilizer that may be adjusted from the surface. The tool includes moveable arms that alternate between collapsed and expanded positions in response to differential fluid pressure between the flow bore and the wellbore. U.S. Pat. No. 5,318,138 discloses an adjustable blade stabilizer including a plurality of blades in an angled track that permits radial movement of the blades. The blades are driven by drilling fluid pressure and are radially limited by a positioning piston. The positioning piston is located in the track in response to a command signal received from the surface. This system is capable of setting the stabilizer diameter to a plurality of discrete diameters (rather than a continuum) and utilizes mud pulse communication to set the stabilizer diameter. The system therefore requires the drilling fluid flow to be stopped and started in order to reset the stabilizer diameter.
While adjustable, rotationally fixed stabilizers are known, for example, as described above, they do not provide for automatic adjustment to the borehole diameter. There exists a need in the art for an automatically adjustable stabilizer and, in particular, an adjustable stabilizer that does not require interruption to the normal drilling process.